The ornamental design for a tread portion of an automobile tire, as shown and described.
FIG. 1 is a front view of a tread portion of an automobile tire, it being understood that the tread pattern repeats uniformly throughout the circumference of the tire;
FIG. 2 is a perspective view thereof;
FIG. 3 is a left side view thereof, the right side view being identical thereto; and,
FIG. 4 is a fragmentary enlarged view showing the details of the tread portion thereof.
The broken lines in the drawing figures illustrate environmental structure only, and form no part of the claimed design.